Deceleration distance over time

[Plebian]

Homework Statement

:[/B]
A person drives a car for 20 seconds in a straight line with an initial velocity of 20m/s. During the entire course of the journey they applied breaks causing the car to decelerate at 5m/s2. How far will it be from the starting point after the given time?

Homework Equations

:[/B]
distance = 20 * 20 + 0.5 * -5m/s^2 * 20^2

The Attempt at a Solution

: [/B]
However since I acquire a negative value, I'm convinced that my working is incorrect. What am I misunderstanding about the deceleration?

 

[BvU]

Hello Plebian,

So what happens to the speed of the car ? As a function of time I mean.

 

[Plebian]

I forgot to mention that I attempted the deceleration over time _{20 * 20 + 0.5 * -5m/s^2 * 20} this rendered a better result that looks like it could be correct. I am on the correct track here?

 

[BvU]

No ! it's not as if you can solve this by trial and error juggling the expressions. The dimensions don't even fit.
Now read post #2 and react to it: what happens if you slam the brakes while moving at 20 m/s and thus reduce your speed with 5 m/s per second ?

 

[Plebian]

No ! it's not as if you can solve this by trial and error juggling the expressions. The dimensions don't even fit.
Now read post #2 and react to it: what happens if you slam the brakes while moving at 20 m/s and thus reduce your speed with 5 m/s per second ?

The velocity of 20m/s would slow down by 5m/s per second.

 

[BvU]

Yes, that's what I wrote. So after 1 second your speed is .., after 2 ..., after 3 ... and after 4 it is zero. Would it then go negative if you keep braking ?

 

[Plebian]

Yes, that's what I wrote. So after 1 second your speed is .., after 2 ..., after 3 ... and after 4 it is zero. Would it then go negative if you keep braking ?

It would remain stationary at that point.

 

[BvU]

Does the expression you use in post #1 reflect that ?

 

[Plebian]

Does the expression you use in post #1 reflect that ?

Not at all, which was why I initially posted this query.

 

[BvU]

So for how long can you use that expression ?

 

[Plebian]

So for how long can you use that expression ?

Point taken.